1. Field of the Invention
The invention relates to a process and a device for continuous production of mineral wool nonwoven fabrics particularly from rock wool by depositing fibers on a collecting conveyor subjected to suction pressure. The invention also relates to processes for the continuous production of felt webs comprising several mineral wool nonwoven fabrics.
2. Discussion of Background
In the production of mineral wool nonwoven fabrics, e.g., from rock wool or glass wool, besides the shredding itself, the formation of the nonwoven fabric as such is an important process step. In this case, as is known, a fiber/gas/air mixture, produced by a shredding unit, for the separation of the fibers, is introduced into a boxlike so-called fall shaft, which in most cases on the bottom side exhibits a collecting conveyor acting as a sort of filter screen, which generally is designed in the form of a gas-permeable rotating plane conveyor belt. In this case under the conveyor belt there is a suction device, which produces a specific partial vacuum.
Now if the fiber/gas/air mixture--which can also contain a binder--strikes the collecting conveyor, the gas/air mixture is suctioned under the collecting conveyor acting as a filter, and the fibers are deposited on the conveyor as nonwoven fabric. On the other hand, if a fall shaft with several consecutively placed shredding units is used to obtain mineral wool nonwoven fabrics, which, in comparison with the first-mentioned device, have higher wool layers, i.e., higher weights per unit area, the already formed partial nonwoven fabric of each previous shredding unit represents an additional flow resistance in connection with each subsequent partial nonwoven fabric for the suction of the gas/air mixture. This means the more shredding units working together in a fall shaft, the higher the flow resistance in the conveying direction of the total nonwoven fabric, and thus the energy consumption of the suction device increases, with whose suction pressure the respective flow resistance must be overcome. To illustrate this principle, reference is made, for example, to U.S. Pat. No. 3,220,812.
Besides the increased energy consumption, such an entire nonwoven fabric formation has the decisive disadvantage that by the relatively high differential pressures resulting in this case between suction device and nonwoven fabric surface the mineral wool nonwoven fabric that is being formed can be compressed so that it leaves the fall shaft precompressed. As a consequence, it is not permissible to fall below preset minimum weights per unit volume of the entire mineral wool nonwoven fabric, i.e., weights per unit volume in wool nonwoven fabrics, e.g., from rock wool, under 25 kg/m.sup.3 can hardly be produced with such devices.
Moreover, the nonwoven fabric formation many times does not proceed homogeneously, so that different weights per unit area can be distributed over the total surface of the nonwoven fabric. Further with such devices with a multiplicity of shredding units there is the disadvantage that with the requirement to produce a mineral wool nonwoven fabric with relatively high weight per unit area, possibly some shredding units must be cut off, as soon as the capacity of the suction device, i.e., its blower performance, is exceeded, to keep the fall shaft capable of functioning.
The circumstance that the nonwoven fabric thickness increases toward the outlet of the fall shaft and the rate of flow at constant suction pressure toward the outlet of the fall shaft decreases, has led in the usual fall shafts even to the suction areas being divided into several zones under the conveyor belt, in fact with increasing suction pressure in the conveying direction. But the problem of high differential pressures and thus the undesired precompressing of the entire nonwoven fabric was not solved with this measure.